纳滤膜难溶盐污染过程的介电监测方法

A method of dielectric monitoring for the nanofiltration membrane fouling process caused by insoluble salt

模拟了难溶盐对纳滤膜的污染过程并对其进行了介电谱解析,探讨了介电谱作为一种监测膜内部或表面污染层信息的方法之可行性.测量了NF90、NF270、NF-3种商用纳滤膜分别被4种难溶盐(CaCO3、CaSO4、BaSO4、SrSO4)污染前后膜/液体系的介电谱,介电谱的差异是来自于因膜孔径的不同而分别在膜孔内和膜表面形成的结垢层;解释了不同膜体系的介电弛豫机制:污染初期是由污染膜和水之间的界面极化和膜表面浓差极化共同决定,而4 h之后主要受界面极化所控制;分析了介电参数随污染时间的变化,发现膜孔径大小以及难溶盐的结晶机制均会对膜污染过程产生影响.进一步,以CaSO4污染纳滤膜为例,利用引入了描述污染了的膜相的常相位角元件(CPE)的等价回路模型,对介电谱进行了理论解析.

The insoluble salt fouling process of nanofiltration membrane was simulated and the dielectric analysis were carried out. The feasibility of dielectric spectra as a noninvasive method to monitor the information on membrane fouling was discussed. The change of dielectric spectra of three industrial nanofhration membranes NF90, NF270 and NF-, before and after polluted by CaCO3, CaSO4, BaSO4, SrSO4, were observed, the difference in the dielectric spectra of these systems was due to the different pollution mode caused by the different membrane pore radius. The membrane NF270 and NF-were fouled in the inner of the membrane pore and NF90 was fouled on the membrane surface where a scaling layer formed. The dielectric relaxation mechanisms of different membrane systems were explained. The relaxation observed at the stage of fouling process was the result of cooperation of interracial polarization （between the fouled membrane and） and the concentration polarization （near the membrane surface） ; the relaxation after 4 hours was mainly controlled by the interracial polarization between the fouled membrane and solution in terrace. Through the analysis of the dependence of dielectrics on the fouling time, it is indicated that both the value of membrane pore and the crystal mechanism of insoluble salts would influence the fouling process. Further, take the membrane fouled by CaSO4 as an example, the dielectric spectra by introducing a constant-phase-element（CPE） to circuit to describe the fouled membrane. Our finding infers that the dielectric spectra method is a powerful method to monitor the pollute process of internal and surface of the membrane non-invasively.